Saturday, February 21, 2015

CRaTER: Lunar Proton Albedo Anomalies

Figure 1. Top: Lunar albedo proton yield map (cylindrical projection) with anomalous yield regions labeled “A” through “E”. Regions A (Mare Serenitatis) and B (Oceanus Procellarum) are both centered near the boundaries of mare regions. Regions C, D and E are all in the highlands on the far side of the Moon. Bottom: Visible global composite image from the Lunar Reconnaissance Orbiter Camera (LROC).

Introduction: Since the launch of the Lunar Reconnaissance Orbiter (LRO) in 2009, the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been mapping albedo protons (~100 MeV) coming from the Moon [1,2].

These protons are produced by nuclear spallation, a consequence of galactic cosmic ray (GCR) bombardment of the lunar regolith. Just as spalled neutrons and gamma rays reveal elemental abundances in the lunar regolith [3-6], albedo protons may be a complimentary method for mapping compositional variations across the Moon’s surface.

Albedo Proton Yield: The CRaTER instrument simultaneously detects albedo protons from the Moon and GCRs arriving from the zenith direction. We divide the number of albedo protons observed over each point on the Moon by the number of GCRs detected over the same location to produce a map of the yield of albedo protons.

We presently find that the lunar maria have an average proton yield which is 0.9% ± 0.3% higher than the average yield in the highlands; this is consistent with some neutron data that shows a similar yield dichotomy due to differences in the average atomic weight between mare regolith and highland regolith [7].

Map Features: There are cases where two or more adjacent pixels (15° × 15°) in the map have significantly anomalous yields above or below the mean.

These include two high-yielding regions in the maria, and three low-yielding regions in the far-side highlands. Some of the regions could be artifacts of Poisson noise, but for completeness we consider possible effects from compositional anomalies in the lunar regolith, including pyroclastic flows, antipodes of fresh craters, and so-called "red spots" which are associated with volcanic domes. We also consider man-made landers and crash sites that may have brought elements not normally found in the lunar regolith.